This invention relates to a two-terminal switch circuit for inclusion in series with a load across the terminals of a voltage supply for periodically energizing the load from the voltage supply. The invention also relates to a road vehicle direction indicator lamp circuit including such a switch circuit -and to a voltage threshold responsive circuit component for use in such a switch circuit.
A two terminal switch circuit of this general kind, for controlling the flashing of the direction-indicating lamps of motor cars or other road vehicles, is shown in FIG. 2 of FR-A-2344177 and is reproduced as FIG. 1 of the accompanying diagrammatic drawings. The circuit comprises a controllable switch connected between said terminals, a resistance and a capacitor connected in series in that order between a given one of said terminals and the other one of said terminals, and a voltage threshold responsive arrangement having an output signal/input signal characteristic which exhibits hysteresis. The voltage threshold responsive arrangement couples the common point of the resistance and the capacitor to a control input of the controllable switch for, in operation, closing the switch in the event of the voltage across the capacitor increasing to above a first threshold value and opening the switch in the event of the voltage across the capacitor subsequently decreasing to below a second threshold value which is lower than the first threshold value. The voltage threshold responsive arrangement has a power supply input for receiving an energizing voltage relative to said other one of said terminals.
FIG. 1 of the accompanying drawings, shows this prior art switch circuit 1 of FR-A-2344177. It has two terminals 2 and 3 respectively and comprises a controllable switch 4 which is constructed as a relay having an energizing coil 5 and contacts 6,7 which are connected between the terminals 2 and 3. A resistor 8 and a capacitor 9 are connected in that order between a given one of the terminals 2 and 3 (terminal 2) and the other one of these terminals (terminal 3). The switch circuit 1 furthermore includes a voltage threshold responsive arrangement 26 having an output signal/input signal characteristic which exhibits hysteresis. Arrangement 26 has a signal input 27 connected to the common point 12 of the resistor 8 and the capacitor 9, a signal output 28 connected to a control input 13 of the relay 4, and a power supply input 29 connected to the terminal 2 for receiving an energizing voltage relative to the terminal 3. Control input 13 is connected to one end of the energizing coil 5 the other end of which is connected to the terminal 3.
The voltage threshold responsive arrangement 26 comprises an npn transistor 10, a pnp transistor 11, resistors 15, 16 and 17, a reservoir (storage) capacitor 18 and a diode 19. Its signal input 27 is connected to the base of the transistor 10. The emitter of transistor 10 is connected to the tap 14 of a voltage divider formed by the resistors 15 and 16, which are connected in series between the power supply input 29 and the terminal 3. The collector of transistor 10 is connected to the base of the transistor 11 via the resistor 17. The emitter of transistor 11 is connected to one electrode of the storage capacitor 18, the other electrode of which is connected to the terminal 3. The one electrode of the capacitor 18 is also connected to the power supply input 29 via the diode 19. The collector of transistor 11 is connected to the signal output 28.
The terminal 3 of switch circuit 1 is connected to ground via a switchable load 20. Load 20 has first and second terminals 43 and 44 respectively and comprises a pair of left indicator lamps 21 and 22 and a pair of right indicator lamps 23 and 24 of a road vehicle. The load 20 also includes a direction indicator switch 25 which is actuable into first, second and third states in which it connects the pair of lamps 21,22 between the terminals 43 and 44, the pair of lamps 23,24 between the terminals 43 and 44, and is open circuit respectively.
In operation a positive potential relative to ground is applied to terminal 2 by a voltage supply 63, for example, the battery of the vehicle. When switch 25 is open, i.e. is in its third state, neither of the two pairs of lamps 21,22 and 23,24 is energized, there is no voltage present between terminal 2 and terminal 3, and the relay contacts 6,7 are open because there is no voltage relative to terminal 3 on the relay control input 13. If switch 25 is now actuated into its first or second states to connect the pair of lamps 21,22 or the pair of lamps 23,24 to the terminal 3, the terminal 3 is then connected to ground through the relevant pair of lamps and therefore becomes negative relative to terminal 2. Storage capacitor 18 charges through diode 19 to the potential difference which is now present between terminals 2 and 3, and the emitter of transistor 10 is taken to a potential intermediate those on terminals 2 and 3 by means of the potential divider 15,16. Capacitor 9 starts to charge through resistor 8. When the voltage across capacitor 9 eventually increases to above a first threshold value equal to the voltage on the emitter of transistor 10 plus the base-emitter threshold voltage of transistor 10, transistor 10 conducts, turning on transistor 11 so that the collector of transistor 11 applies a positive voltage to the control input 13 of relay 4. The relay contacts 6,7 close, connecting terminal 2 to terminal 3 and thereby energizing the relevant pair of lamps 21,22 or 23,24. Although there is no longer any potential difference between terminals 2 and 3, transistors 10 and 11 continue to be supplied with operating current from the storage capacitor 18. Closure of the relay contacts 6,7 effectively connects the two potential divider resistors 15 and 16 in parallel so that the positive potential on the emitter of transistor 10 relative to terminal 3 is reduced, thereby increasing the forward base-emitter bias of transistor 10 and causing the relay contacts 6,7 to be held closed.
Capacitor 9 now starts to discharge through resistor 8 and the closed relay contacts 6,7. When the voltage on capacitor 9 eventually decreases to below a second, lower, threshold value equal to the reduced emitter voltage of transistor 10 plus the base-emitter threshold voltage of transistor 10 transistor 10 turns off, turning off transistor 11 and thereby removing the positive voltage from the control input 13 of relay 4. The relay contacts 6,7 open so that the lamps of the relevant pair 21,22 or 23,24 are extinguished. The opening of the contacts 6,7 also removes the parallel connection of the potential divider resistors 15 and 16 so that the emitter voltage of transistor 10 increases relative to terminal 3, thereby reducing the base-emitter voltage of transistor 10 still further and causing the ralay contacts 6,7 to be held open. Capacitor 9 now starts to change again through resistor 8 and the cycle repeats.
A disadvantage of the known switch circuit is that it is necessary to provide, in addition to a timing capacitor 9, a storage capacitor 18 to supply operating current to the transistors 10 and 11 at times when the relay contacts 6,7 are closed.
It is an object of the invention to provide a simplification in this respect.
According to a first aspect of the invention there is provided a two-terminal switch circuit for inclusion in series with a load across a voltage supply for periodically energizing the load from the supply comprises a controllable switch connected between said terminals, a resistance and a capacitor connected in series in that order between a given one of said terminals and the other one of said terminals, and a voltage threshold responsive arrangement the output signal/input signal characteristic of which exhibits hysteresis, said voltage threshold responsive arrangement coupling the common point of the resistance and the capacitor to a control input of the controllable switch for, in operation, closing the switch in the event of the voltage across the capacitor increasing to above a first threshold value and opening the switch in the event of the voltage across the capacitor subsequently decreasing to below a second threshold value which is lower than the first threshold value, said voltage threshold responsive arrangement having a power supply input for receiving an energizing voltage relative to said other one of said terminals. In accordance with the present invention such a circuit is characterized in that said power supply input is connected to said common point.
It has now been recognized that the voltage threshold responsive circuit can be constructed so as to allow it to be powered from across the capacitor, thereby making it unnecessary to provide a separate storage capacitor.
The controllable switch is preferably a transistor switch, preferably a Metal-Oxide-Semiconductor (MOSFET) switch. Transistor switches in general require lower control currents than, for example, relays. As these control currents have to be supplied from the capacitor, implementing the controllable switch as a transistor switch, preferably a MOSFET switch, can allow a lower value to be chosen for the capacitor than would be required if, for example, the controllable switch were implemented as a relay.
The voltage threshold responsive arrangement may, for example, comprise a resistive voltage divider connected across said capacitor and a thyristor structure having a control input connected to a tap on said resistive voltage divider, an output connected to the control input of the controllable switch and a common point which is connected to said power supply input.
As an alternative the voltage threshold responsive arrangement may comprise first and second resistive voltage dividers connected across said capacitor, and a set-reset flip-flop having a power supply input connected to the common point of the resistance and the capacitor, a set input connected to a tap on the first voltage divider, a reset input connected to a tap on the second voltage divider and an output connected to the control input of the controllable switch.
As another alternative the voltage threshold responsive arrangement may comprise an operational amplifier having a power supply input connected to the common point of the resistance and the capacitor, a non-inverting input connected to the common point of the resistance and the capacitor via a resistance, an inverting input connected to a reference voltage source, an output connected to the control input of the controllable switch, and a resistance connecting its output to its non-inverting input.
The resistance which is connected in series with the capacitor may be constituted by a constant current source. Constituting the resistance in this way can render the frequency at which the load is energized in operation substantially independent of the terminal voltage of the voltage supply.
As an alternative the resistance which is connected in series with the capacitor may be constituted by a controllable current source and the switch circuit may include a current sensing arrangement for sensing the magnitude of the current which flows through the controllable switch when the controllable switch is closed, the current sensing arrangement having an output coupled to a control input of the controllable current source for controlling the output current of the controllable current source in such manner that this output current increases with a decrease in the magnitude of the sensed current. Such a construction can result in the frequency with which the load is energized in operation varying inversely with the resistance of the load, which can be advantageous if, for example, the load comprises a plurality of road vehicle direction indicator lamps connected in parallel with each other. Such a construction may be implemented, for example, by employing a MOSFET structure as the controllable switch, this MOSFET structure having an additional source region which is connected to said other one of said terminals via a resistance, said additional source region being coupled to the control input of the controllable current source via a sample-and-hold circuit.
The invention also provides a road vehicle direction indicator lamp circuit including a two-terminal switch circuit according to the first aspect connected in series with a load across a battery included in the vehicle, the load having first and second terminals and comprising a pair of left indicator lamps, a pair of right indicator lamps and a direction indicator switch which is actuable into first, second and third states in which it connects the pair of left indicator lamps between the first and second terminals of the load, the pair of right indicator lamps between the first and second terminals of the load, and is open circuit respectively.
The invention also provides a voltage threshold responsive circuit component for use in a two-terminal switch circuit according to the first aspect in which the controllable switch is a transistor switch, said component having a first terminal for use as said given one of said terminals, a second terminal for use as said other one of said terminals and for connection to one electrode of the capacitor, and a third terminal for connection to the other electrode of the capacitor, the component comprising said resistance connected between the first and third terminals of the component, said transistor switch having its main current path connected between the first terminal of the component and the second terminal of the component, and said voltage threshold responsive circuit coupling the third terminal of the component to the control electrode of said transistor switch, said voltage threshold responsive arrangement having its power supply input connected to the third terminal of the component.